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Planetary News: Pluto (2006)Three Plutonic Moons Formed in Single Cataclysmic Event, Scientists SayBy Amir Alexander24 February 2006 As New Horizons speeds along on its long journey to Pluto, scientists on Earth are finding out more and more about its target destination. The Pluto-Charon system, which until recently had been considered a double planet, now has four known members. The newcomers, known for the moment as S/2005 P1 and S/2005 P2, were discovered in the summer of 2005 orbiting Pluto beyond Charon. At between 55 and 160 kilometers (33 and 96 miles) across, S/2005 P1 is a tiny fraction of the size of Charon's 1,200 kilometers (720 miles), and S/2005 P2 is even smaller, at 45 to 130 kilometers (27 and 78 miles). Nevertheless, scientists believe that their presence, location, and orbit, can tell us a great deal about the evolution of the Plutonic system. In a paper published in the February 23, 2006 issue of Nature magazine a group of scientists led by Alan Stern of the Southwest Research Institute (SwRI) and Hal Weaver of the Johns Hopkins Applied Physics Laboratory argue that the evidence seems to show that the two small moons were formed by the same cataclysmic event that forged the giant Charon. Scientists have long believed that Charon was created when a massive planet-sized body slammed into Pluto early in its history, sending massive amounts debris into orbit around the planet. Over time the rocky debris accreted and formed Charon. This scenario is of great interest to planetary scientists since it mirrors the way the Earth's Moon was created as well. The paper's authors, which also include A. J. Steffl, W. J. Merline, E. F. Young, L. A. Young, and J. R. Spencer from SwRI, M. J. Mutchler from the Space Telescope Science Institute, and M. W. Buie from the Lowell Observatory, point to several unusual characteristics of the new-found moons. Both appear to orbit Pluto in near-circular paths, on the same orbital plane as Charon. Astronomers know this because when the moons were first detected near Pluto in May 2005, they predicted the future positions of the two based on these very assumptions. When the moons promptly showed up almost exactly where they were predicted, the scientists concluded that their initial assumptions were almost certainly correct. In addition, S/2005 P1 and S/2005 P2 appear to be in orbital resonance with Charon, meaning that the ratio of their periods and that of Charon can be expressed as a simple integer ratio, 2:3 or 3:4, for example. All of these factors – the circular paths, the orbital plane, and the orbital resonance, seem to point to a single conclusion: S/2005 P1, S/2005 P2, and Charon have a common origin.
"The idea that Pluto's small moons and Charon resulted from a giant impact now seems compelling" said Robin Canup, director of SwRI's Space Studies Department, adding that "if I had to place bets, I'd go with an impact origin." "But," she cautioned. "I don't think we can say that firmly yet." Further tests of this scenario, said co-author Weaver, will come from better orbital data and more information about the moons' composition. One of the interesting predictions coming out of the study is the possibility that Pluto, at certain times in its history, may possess rings. These, the study suggests, would form when the two small moons are struck by relatively large space rocks, sending large amounts of debris into space. Orbital calculations show that in these events, the debris would not escape into space or settle back onto the surface of the moons, but most likely enter into orbit around Pluto. "If Pluto's small moons generate debris rings from impacts on their surfaces, as we predict," said co-author Merline, "it would open up a whole new class of study." It would constitute the first time, he explained, that a ring system is found around a solid body rather than a gas giant planet. In July of 2015, New Horizons will fly right through the Pluto system, taking close range pictures and measurements of all four bodies. The spacecraft's Long Range Reconnaissance Imager (LORRI) camera will take high-resolution images of the two moons, resolving features as small as 550 meters wide. "We're getting four fascinating objects for the price of two" said co-author Weaver. "The opportunity to explore the 'bookends' of Kuiper belt object size distribution, with Pluto and Charon at one end and P1 and P2 at the other, is an unexpected treat." |
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